Polymeric films, especially oriented, heat shrinkable films, have gained wide acceptance in the food industry for use in making bags for packaging food products. Typically, bags formed from such films are provided to food processors with one end open to form a bag mouth. After the product, e.g. a subprimal of fresh red meat, is inserted into the bag mouth, air is normally evacuated from the bag, the mouth of the bag is closed, e.g. by heat sealing, and heat is applied to initiate shrinkage of the bag about the enclosed product.
Commercially useful bags must satisfy many requirements imposed by both the bag producer and the bag user. The bag should survive physically intact the process of being filled with a product, evacuated, sealed, and heat shrunk. The bag should also be strong enough, i.e. abuse resistant, to survive the normal material handling involved in moving the bagged product along a distribution system.
The bag also must often serve as a barrier to the infusion of gas, especially oxygen, from the surrounding environment. Typically, the structure of the bag will include an effective barrier to the infusion of oxygen since oxygen is well known to cause spoilage of food products.
A variety of different films have been used to form bags suitable for use in the food industry. One such film is disclosed in U.S. Pat. No. 3,741,253 to Brax et al., incorporated by reference herein in its entirety. This film has an inner layer of a cross-linked ethylene /vinyl acetate polymer, a directly joined middle layer of a polymer of vinylidene chloride and a third layer directly joined to the middle layer.
The film discussed above, when used in product packaging operations, is subject to two distinct performance concerns.
The first concern results from the solubility of the outer layer of the film resulting from the exposure of the film to grease and heat. During product packaging operations the film may be exposed to grease, for example, resulting from mineral oil used in cleaning the packaging equipment and/or naturally occurring grease generated from the product being packaged. When grease contacts the outer layer of the film and the film is subsequently heated during the shrinking operation of the film around the product, the integrity of the outer layer of the film can be reduced due to solubility.
The second concern is the potential for imperfect sealing of the package due to the sticking of a portion of the bag's outermost layer to a seal bar used to close the bag with associated transfer of polymer to the seal bar. This is generally exacerbated by inadequate interlayer adhesion. Eventually, there can be sufficient build-up of material on the sealing bar to interfere with proper sealing.
One solution to the foregoing concerns is to cross-link the outer layer of the film. Cross-linking renders the material insoluble to grease, enhances the interply adhesion, and improves the cohesiveness and resistance to flow of the heated outer layer during sealing. However, in typical film production processes such as extrusion coating, exemplified in Brax et al. '253, the outer layer is not irradiated and therefore not crosslinked, and the innermost layer is irradiated and crosslinked.
Definitions
"Crosslinked" and the like are used herein to denote a process to increase the polymer molecular weight, as well as the resultant state of the material after such a treatment. The use of ionizing radiation for crosslinking polymeric films is disclosed in U.S. Pat. No. 4,064,296, to BORNSTEIN, et. al., which is hereby incorporated in its entirety, by reference thereto. In the irradiation process, the film is subjected to an energetic radiation treatment, such as corona discharge, plasma, flame, ultraviolet, X-ray, gamma ray, beta ray, and high energy electron treatment. Radiation dosages are referred to herein in terms of the kilogray (kGy). A suitable radiation dosage of high energy electrons is in the range of 1 to 250 kGy, more preferably about 40-140 kGy, and still more preferably, 80-120 kGy. Preferably, irradiation is carried out by an electron accelerator and the dosage level is determined by standard dosimetry methods. Other accelerators such as a Van de Graaff or resonating transformer may be used. Crosslinking via irradiation is not limited to ionizing radiation as other energetic methods such as ultraviolet radiation may be used. The most preferred amount of radiation is dependent upon the film and its end use. Crosslinking can also be effected by chemical means.
"Non-crosslinked" and the like refers to materials that have not been crosslinked by exposure to an energetic radiation treatment, or chemical treatment, of the kind disclosed herein.
"Film" herein means a film, laminate, sheet, web, or the like which can be used to make a bag, and/or package an article.
"Centerfolded film" and the like is used herein to denote the process and condition of an annular film that has been longitudinally slit, resulting in two individual webs in face to face, congruent alignment, with the fold forming one of the lateral sides of the film, and the web edges forming the other lateral side of the film.
As used herein, the term "oriented" refers to a polymer-containing material which has been stretched at an elevated temperature (the orientation temperature), followed by being "set" in the stretched configuration by cooling the material while substantially retaining the stretched dimensions. Upon subsequently heating unrestrained, unannealed, oriented polymer-containing material to its orientation temperature, heat shrinkage is produced almost to the original unstretched, i.e., pre-oriented dimensions. Oriented films in accordance with the invention are characterized by a free shrink of at least 5% at 180.degree. F., in either the longitudinal direction, the transverse direction, or both the longitudinal and transverse directions, as determined by ASTM D 2732.